1. Field of the Invention
The invention relates to a source driver, and more particularly to a source driver capable of shifting voltage levels of driving voltages.
2. Description of Related Art
In current information era, the information broadcasting media and all kinds of electronic displays have been widely applied to the industry apparatus or the home version apparatus, and thus, the electronic displays have become more and more important. These electronic displays are also continuously developed to satisfy all kinds of requirements in the information society.
Generally, the electronic displays display and transmit all kinds of information to the user who uses it. That is, these electronic displays convert electric information signals to optical information signals which can be visually recognized by the user.
In current display apparatus or system, such as the cathode-ray tube (CRT) or the liquid crystal display (LCD), the relationship between the input voltage and the display output is not a linear one, and the relationship therebetween is described through the gamma curve. For the LCD, through the gamma curve, the output voltages (i.e. the gamma voltages) corresponding to each gray level can be found. By using these gamma voltages to control the LCD panel to display exact gray levels, the LCD can display an exact image.
In order to improve the display effect of the LCD, one pixel in the LCD panel is divided into two sub-pixels. Due to the configuration of the circuit, the voltage levels of the common voltages of the two sub-pixels may be different. In this case, by using the same gamma voltage to control the LCD panel, the two sub-pixels may display the different effects, thereby affecting display quality. Accordingly, if the different sub-pixels displaying the same effect is required, the voltage levels of the outputted gamma voltages may be different. That is, when some pixels display the same effect, receiving a shifted gamma voltage is required.
FIG. 1 is a circuit diagram of a conventional source driver. Referring to FIG. 1, in the source driver 100, the channel buffer 110 is used to shift the reference voltage GMAH by an offset voltage ΔV, and output it after that, wherein the channel buffer 110 is configured by an operational amplifier and a plurality resistors R, of which the function is similar to that of a subtractor. Similarly, the channel buffer 120 is used to shift the reference voltage GMAL by an offset voltage ΔV, wherein the channel buffer 120 is configured by an operational amplifier and a plurality resistors R, of which the function is similar to that of a subtractor. After being applied with the shifted reference voltages GMAH and GMAL, the resistors AR1-AR64 divide them to output the shifted gamma voltages AV0-AV63. The digital-to-analog converter 130 selectively outputs one of the gamma voltages AV0-AV63 as the driving voltages.
However, the conventional source driver is required simultaneously shifting the reference voltages GMAH and GMAL. That is, the channel buffers 110 and 120 need to work simultaneously so as to synchronize the voltage levels of the gamma voltages AV0-AV63. In addition, duce to the phenomenon of OP (operational amplifier) deviation of the channel buffers 110 and 120, the voltage drop between the output voltage of the channel buffer 110 and the reference voltage GMAH may be not equal to the offset voltage ΔV, and the voltage drop between the output voltage voltage of the channel buffer 120 and the reference voltage GMAL may be not equal to the offset voltage ΔV. Therefore, the voltage levels of the gamma voltages AV0-AV63 may not shifted equally.